Most power semiconductor devices, e.g., high-voltage P-I-N diodes, and power transistors, e.g., power metal-oxide-semiconductor field-effect transistors (MOSFETs) and Insulated Gate Bipolar Transistors (IGBTs), have been typically fabricated with silicon (Si) semiconductor materials. Silicon carbide (SiC) power devices have also been used. III-N devices are attractive semiconductor devices to carry large currents and support high voltages, and provide very low on resistance, high voltage operation, and fast switching times.
Some III-N devices, e.g., high electron mobility transistors (HEMTs) and bidirectional switches (also known as four quadrant switches or FQSs), may be depletion-mode (or D-mode) or normally-on devices, e.g., devices with a negative threshold voltage. That is, the devices are in the ON state unless a sufficiently negative voltage is applied to the gate electrode relative to the source or power electrode. In many applications, in particular in power switching circuits, it is desirable to utilize enhancement-mode (or E-mode) devices, e.g., devices with a positive threshold voltage, as this can simplify the form of the signals applied by a gate-drive circuit to the device and can prevent accidental turn on of the device in case of device or circuit failure. Reliable fabrication and manufacturing of high-voltage III-N E-mode devices has proven to be very difficult. In some cases, a high-voltage III-N D-mode device and a low-voltage silicon-based E-mode device are combined together to form a hybrid device, which may achieve the same or similar output characteristics as a single high-voltage III-N E-mode device.